High-silica glasses have wide commercial application, in part because of their refractory properties, low thermal expansion, high thermal shock resistance, good chemical resistance, and good dielectric insulating properties when low in alkali-oxides. Recently, very pure high-silica glasses have been fabricated, and such glasses have been successfully applied to produce extremely low-loss optical fibers for communication systems.
High-silica glasses are made primarily by melting of appropriate starting materials, by the sol-gel method, or by depositing the glassy product of appropriate gas phase reactions. This application is concerned only with the second method and a subgroup of the third method, the subgroup comprising the so-called Vertical Axial Deposition (VAD) method, and the Outside Vapor Phase Oxidation (OVPO) method. The glass-forming methods of concern in this application have the common feature that a porous intermediate body exists at some point during glass manufacture, and that this intermediate body is at some later stage heat treated to result in a dense glass body. The glass body is then typically re-heated to at least a sintering temperature, and may be subjected to a shape-changing treatment, e.g., fiber drawing. Such heating treatment of the glass body will be referred to herein as "re-heating".
Although the fusion method of glass fabrication is probably the most widely used method, the sol-gel method of producing high-silica glasses can actually have significant advantages over the former. See, for instance, commonly assigned U.S. Pat. No. 4,419,115, incorporated herein by reference. For a description of the VAD and the OVPO methods, see T. Izawa et al, 1977 International Conference on Integrated Optics and Optical Fiber Communication, (Tokyo, Japan), pp. 375-378; and U.S. Pat. No. 3,806,570 issued to J. S. Flamenbaum et al, respectively.
Several variants of the sol-gel method for forming high-silica glass are known in the art. Among these are processes comprising hydrolysis and polymerization of a metal alkoxide, and processes that use particulates such as fumed silica in sol-formation. Alkoxide processes are described, for instance, in the chapter by S. Sakka in Treatise on Materials Science and Technology, Vol. 22, M. Tomozawa and R. H. Doremus, editors, Academic Press, 1982, pp. 129-167. For an example of the particulate method, see, for instance, U.S. Pat. No. 4,042,361, which discloses a method of densifying fumed silica involving drying a flowable sol of silicate to form a fragmented solid which is then calcined, milled to provide a slip for casting silica articles which are then fused by heating to temperatures above the liquidus.
Fabrication of high-silica glass articles often comprises a manufacturing step in which the prepared glass is heated to a temperature above the softening temperature. For instance, optical fiber is typically produced by drawing from a glass body, generally called a preform, the drawing typically requiring heating part of the preform to a temperature of the order of 2200.degree. C. It has been observed in the prior art that glass produced by a sol-gel method is subject to bubble formation or reboil, also sometimes called bloating, during such high temperature treatment. Bubble or pore formation can also occur in VAD and OVPO-produced glass. See, for instance, K. Ishida et al, Fiber and Integrated Optics, Vol. 4, No. 2, Crane, Russak & Co. (1982), pp. 191-202.
Bubble formation is a highly undesirable phenomenon that typically requires rejection of affected articles. For instance, bubbles in such articles as optical fiber, lenses, prisms, or other optical elements result in light scattering that makes such articles generally unacceptable.
U.S. Pat. Nos. 3,954,431 and 4,011,006, co-assigned with this, teach, inter alia, that the inclusion of small quantities of GeO.sub.2 in borosilicate glass suppresses bubble formation. It is also known that execution of an exacting heating routine during sintering, or sintering in a He-atmosphere, can reduce bubble formation during subsequent higher temperature manufacturing steps.
However, due to the economic potential of high quality sol-gel, VAD, and OVPO high-silica glass, the availability of a broadly applicable, simple, and reliable method for eliminating bubble formation would be highly desirable. This application discloses such a method.
The prior art knows many elements and compounds that can be incorporated into high-silica glass to produce changes in the physical characteristics of the glass. For instance, germanium is an important dopant in glass for optical fibers, since it results in an increase of the refractive index of silica without causing optical absorption at wavelengths of current interest. Another well-known dopant is fluorine, which lowers the refractive index of silica, in addition to markedly decreasing its viscosity and lowering its glass transition temperature. See, for instance, W. Eitel, Silicate Sciences, Vol. VIII, paragraph 95, page 54, Academic Press, 1976; K. Abe, Proceedings of the Second European Conference on Optical Fiber Communications, IEE, 59-61, Paris, France, 1976; and K. Rau et al, Topical Meeting on Optical Fiber Transmission II, Williamsburg (1977), pp. TUC 4-1 to TUC 4-4.